Mounting units are known, for example, from EP 0 272 642 B1. In that reference, a sleeve-shaped holding part not only serves as an anti-loosening lock, but also as a carrier for an intermediate part made of a rubbery material inserted between the mounting unit and the holding part. The intermediate part is further designed to insulate against noise emanating from the structure. The rubbery consistency of the intermediate part and the thin-wall design of the holding part permit their radial, elastic expansion, thus making it simple to produce an anti-loosening lock. For the above purpose, the sleeve has at the upper end of its throughbore a collar that slightly necks to form an axial interlock or function as part of the anti-loosening lock. During the axial insertion of the fixing part into the sleeve, the collar is expanded elastically in a radial direction by two radial projections on the fixing part which extend past the transverse cross-section of a shaft of the fixing part. Once the radial projections have passed, the collar bounces back radially toward the inside of the sleeve into the undercutting area or space of the shaft. The above completes the fixation safety device. However, the aforementioned premises for a radial expandability through the elasticity of the sleeve collar limit the possible uses for the above arrangement.
The trend in assembly technology is toward working with the highest possible degree of prefabrication. Whereas in the past, for example, screws were not screwed in until the final assembly of a mounting part--e.g. a casing lid--it is now expected that this mounting part is delivered with the necessary mounting screws already in place as one mounting unit, ready for final assembly. The final assembly can then be produced in a comparably much shorter amount of time, and, if necessary, even automatically with a robot. In doing so, it must be ensured that the screws are connected to the mounting unit such that they cannot become loose. At the same time, the selection of the material for the mounting part in most cases cannot be a function of whether it is suitable for the pre-fixation of the fastening screws to the mounting part to form an anti-loosening lock.
Furthermore, it must be guaranteed that the fixing ends, meaning the threads for the screws, remain undamaged between preliminary assembly and final assembly. In particular, this concerns the transport and storage of mounting parts equipped with fixing parts. An additional problem is that the mounting part has several screws around the circumference, as is the case, for example, with casing lids. In that case, precautions must be taken during the assembly that the mounting part is not skewed when the screw threads do not simultaneously engage in their counter-threads. This would impede further assembly or make it impossible. This applies in particular when, due to production tolerances on the base component, for example, an engine block, the counter-threads which cooperate with the thread of the fixing part are not positioned precisely, that is, if their axes are offset with respect to the axes of the threads of the fixing part.
Such a mounting unit is known from U.S. Pat. No. 3,465,803 A. In this reference, the axially effective captively securing arrangement between fixing part and holding part is accomplished through plastic deformation of the holding projection which is oriented radially inward on the holding part in the manner of a circumferential collar. For this purpose, an annular flange is provided between head and shaft of the fixing part, with the annular flange projecting radially beyond the shaft; at the end of the joining process, the annular flange acts in the manner of a collar radially inwardly oriented holding projection of the holding part with its shoulder which is adjoining the shaft and plastically deforms the holding projection such that its effective inside diameter is made smaller and it is therewith moved into a position in which it extends behind the fixing projection of the fixing part configured there as a screw thread. But during this process the annular flange creates a connection, seen radially, between fixing part and holding part in the final joining position, which connection only allows a slight radial play between these parts.
In addition, the plastic deformation of the holding projection for the creation of the captively securing arrangement requires a considerable joining pressure. To transmit this joining pressure, the annular flange below the head of the fixing part is configured to be relatively massive. The holding part must have a great wall thickness so as to prevent the holding part from being deformed under the necessary high joining pressure. During assembly, the lacking radial play makes it impossible to compensate for hole inaccuracies.
Furthermore, such mounting units, for example, are known from EP 0 272 642 B1. There, the sleeve-shaped holding part not only serves as anti-loosening lock, but also as carrier for an intermediate part made of a rubbery material that is inserted between mounting unit and holding part and is designed to insulate against structure-born noise. The rubbery consistency of this intermediate part and the thin-wall design of the holding part permit their radial expandability, thus making it simple to produce an anti-loosening lock. For this, the sleeve has at the upper end of its throughbore a collar that slightly necks in the clear opening to form an axial interlock or function as part of the anti-loosening lock. During the axial insertion of the fixing part, this collar is expanded elastically in radial direction by two radial projections on the fixing part, which extend past the shaft cross-section. Once the radial projections have passed, the collar bounces back radially toward the inside, into the undercutting area of the shaft for the fixing part. This completes the anti-loosening lock. However, the aforementioned premises for a radial expandability through elasticity of the sleeve collar limit the possible uses for this solution.